A conventional electromagnetic damper includes a cylinder and an outer body constructed to extend and contract relative to each other. The movement of the cylinder makes a nut fixed to the cylinder rotate a ball shaft formed with screw threads, so that rotation of a motor connected to the ball shaft generates an induced electromotive force. By regulating an electrical current flowing in the motor, a desirable damping force is obtained.
An electromagnetic damper control device for regulating this electrical current as proposed in JP 2001-311452 A, raises the induction voltage in the electromagnetic damper by changing the duty rate for switching a transistor that switches the electrical current output from the motor, so that the desired damping force can be obtained in the electromagnetic damper.
However, in order to operate a regulating circuit for this type of control, a supply of external electrical power to the regulating circuit is required in the above described conventional electromagnetic damper control device. In other words, the conventional electromagnetic damper has a problem that damping effect could not be obtained in a state where the power supply is off. Moreover, since the duty rate of the switching transistor is set by a control program based on the voltage generated in the motor, it is not easy to change the damping force characteristics of the electromagnetic damper.
It is therefore an object of this invention to provide an electromagnetic damper control device capable of arbitrarily changing the damping force characteristics of the electromagnetic damper without requiring a supply of external electrical power.